A tubular article, such as electrical insulation tubing, can be formed by heat treatment of a preform of the article. The tubular article can be formed, for example, from a thermosetting plastic preform that is molded into a final article by heat curing. The preform may be a single constituent material or a composite of constituent materials. The single constituent material, or composite of constituent materials, may be electrically conductive or non-electrically conductive. Composite materials can include, for example, powered particles mixed in a binder, a fiber reinforced composite or a discrete multi-layered composite that may include a mix of electrically conductive and non-electrically conductive layers. Alternatively the preform may be heat treated to form an adhesive that is subsequently applied to another material.
A tubular mandrel may be used in heating systems for curing or drying a preform loaded onto the outer surface of the mandrel. Mandrels can vary significantly in size, for example, from the size of a thin needle to eight inches in diameter and twenty feet in length. One type of known heating system for heat curing (or drying) a preform is a tubular mandrel and a hot fluid medium such as a heated liquid (for example steam), thermal oil or polymer that flows inside of the tubular mandrel to heat the mandrel, which, in turn, heats by conduction, the preform loaded on the outside surface of the mandrel. After a substantial amount of the hot medium flows through the tubular mandrel there can be an appreciable deflection of the mandrel. This deflection may be as much as five to seven inches and can result in shape distortion. In order to compensate for the effects of gravity and provide a high quality tubular product with a consistent wall thickness, the mandrel can be rotated. Circulating a large volume of a hot fluid medium through the tubular mandrel presents safety concerns; raises obvious maintenance issues (for example leakage of the hot fluid); and makes the entire curing or heating system cumbersome.
Another type of known heating system for curing or drying a preform comprises an electric induction scanner with an induction coil that surrounds the preform on the mandrel and inductively heats a metallic mandrel by induced eddy current. However axial deflection (wobbling) of the mandrel during rotation requires an appreciable “coil surface to mandrel surface” gap that negatively affects the electrical efficiency of the induction heating system, as well as resulting in poor product quality and process sensitivity.
One object of the present invention is uniformly and efficiently heat treating a tubular shaped preform by resistively heating a longitudinally oriented electrical conductor upon which the tubular preform has been loaded.
Another object of the present invention is to improve the robustness and safety of equipment for heating tubular shaped preforms and articles formed therefrom.
Another object of the present invention is to avoid generating stray electromagnetic and electrostatic fields resulting from application of electrical energy for resistance heating of the mandrel.